Various techniques for measuring angular measurements of radiation beams relative to a reference plane have been known and used in the past. Typical of these are the ones known as the quad cell technique and the knife edge concept, both of which have been practiced for a number of years. The state of the art requirement for measuring small angular deviations with devices of this type is 0.1 arc second. In all conventional techniques now practiced in this field, none has been able to meet this requirement because each such conventional technique has been carried out with structural components which are not capable of providing the necessary precision for angular measurements of such small magnitudes. As a result, a need has existed for some time for improvements in such techniques so that the state of the art requirement can be attained without the use of complex and expensive structural features.